1. Field of the Invention
The present invention relates to medium access control methods and systems. More specifically, the present invention relates to a medium access control method to be used in a network system including a plurality of terminals accessing each other for allowing a specific terminal (control station) to control access of all of the other terminals (terminal stations) in a time division manner, and a medium access control system using the method.
2. Description of the Background Art
In order to achieve a communication network system including a control station for controlling access to a network and terminal stations for accessing the network under the control of the control station, various wireless LAN systems have been put into practical use. Examples of widely-available wireless LAN systems include those complying with the IEEE802.11b standard using the 2.4 GHz band and those complying with the IEEE802.11a standard using the 5 GHz band. Furthermore, the IEEE802.11e standard, which includes the concept of Quality of Service (QoS), has been under planning.
Wireless LANs complying with the IEEE802.11 series define a media access scheme called Point Coordination Function (PCF), which is one of medium access schemes. In this scheme, a control station called Access Point (AP) transmits a polling frame to a terminal station called Station (STA), thereby allowing the terminal station to perform transmission. In detail, refer to IEEE Std 802.11, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications”.
FIG. 24 depicts one example of a wireless LAN network including one access point AP and three stations STA1 through STA3 in which PCF medium access is performed. In the example of FIG. 24, AP first transmits a polling frame 501 to STA1 for allowing transmission of a data frame. After STA1 finishes transmitting a data frame 502, AP transmits a polling frame 503 to STA2. In response, STA2 transmits a data frame 504. Thereafter, the same sequence is performed between AP and STA3.
On the other hand, IEEE802.11e defines a medium access scheme called Hybrid Coordination Function (HCF), which is a combination of PCF with the concept of QoS. In detail, refer to Consumer Communications and Networking Conference, 2004. CCNC 2004. First IEEE, 5-8, January 2004, “A QoS scheduler for IEEE 802.11e WLANs.
FIG. 25 depicts one example of a wireless LAN network including one access point AP and three stations STA1 through STA3 in which HCF medium access is performed. In the example of FIG. 25, AP first transmits a polling frame 511 to STA1. At this time, a time period during which STA1 can occupy the medium is written in the polling frame. This time period during which a specific STA can occupy the medium is called Transmission Opportunity (TXOP). Within the time period specified by TXOP, STA1 can perform data transmission any number of times (a plurality of pieces of data 512). Upon expiration of TXOP, AP specifies another TXOP to transmit a polling frame 513 to STA2. By repeating this series of processes, the time period during which each STA can occupy the medium can be specified.
On the other hand, in another technology, a control station transmits a frame (timing signal) including communication timing information to each terminal station in the network, thereby specifying a time period during which each terminal station can occupy a communication medium. Examples of such technology are disclosed in Japanese Patent Laid-Open Publication No. 9-205454 (hereinafter, a first document) and Japanese Patent Laid-Open Publication No. 2001-333067 (hereinafter, a second document).
Particularly, in the second document, each terminal station in the network performs communication based on the timing defined by the control station in a timing signal while performing carrier detection, thereby further improving transmission efficiency. Specifically, in the second document, as shown in an example of FIG. 26, a transmission start time of each station is defined based on the order of transmission. In FIG. 26, it is defined that a terminal station H in the first transmission order starts transmission immediately after receiving the timing signal, a terminal station I in the second transmission order starts transmission 30 msec thereafter, a terminal station J in the third transmission order starts transmission 120 msec thereafter, and a terminal station K in the fourth transmission order starts transmission 150 msec thereafter. Furthermore, as exemplarily shown in FIG. 27, an idle time until each terminal station starts transmission is defined for each terminal station in accordance with the transmission order so as to specify the transmission order even if a communication time of a specific terminal station goes over the scheduled communication time. In FIG. 27 at left, idle times for the terminal stations H through K in the first through fourth transmission orders are set as 5 msec, 10 msec, 15 msec, and 20 msec, respectively.
However, in this scheme of providing idle times, a terminal station in a lower transmission order is set with a long idle time. This causes reduction in transmission efficiency. Therefore, in the second document, if a specific terminal station has successfully completed normal transmission based on the timing signal, the idle times of the other terminal stations lower in order than the specific terminal station are moved up one by one for resetting, thereby avoiding reduction in transmission efficiency in normal transmission (refer to FIG. 27 at right).
As described above, in the conventional communication network system, an available time period for transmission is defined for each terminal station, thereby achieving highly-efficient data transmission.
However, the above-described scheme of performing polling has two problems. Firstly, a polling frame has to be transmitted to each terminal station separately, thereby increasing overhead. Particularly, when transmission data of each terminal station is small or when it takes relatively long time to transmit a polling frame, transmission efficiency is sharply decreased because polling frame transmission has to be performed a large number of times. In IEEE80211, a plurality of transmission modes are defined. However, in order to allow the receiving terminal to detect the transmission mode in use, the transmission mode of the lowest speed is always used for the header portion of a polling frame. An increase in the number of times the polling frame transmission not accompanying data transmission is performed means an increase the number of times the low-speed header transmission is performed. Consequently, transmission efficiency is decreased.
Secondly, when TXOP in IEEE802.11e is fixedly allocated, even if a terminal station uses only a part of the allocated time period for communication, for example, another terminal station cannot use the unused time period (idle time) for communication. In wireless communication (e.g. IEEE802.11e) and electrical line communication using the household power line, the state of the transmission path is not stable, thereby frequently causing transmission path errors in the transmitted frames. In one scheme for mitigating such transmission errors, a sender station transmits a predetermined amount of data in advance, a receiver station stores the data using a buffer or the like, and a frame having an error is retransmitted while processing the data in the buffer in the receiver station. However, this scheme is useless if data delay occurs more than the capacity of the buffer. This is because, even if the amount of data stored in the buffer of one terminal station is reduced after processing some portion of the data and it is desired to recover the amount of data in the buffer, a time period allocated to another terminal station cannot be used for this recovery.
Furthermore, the scheme disclosed in the second document has the following problem. That is, the idle time set in the second document is an extra time which is originally not required if normal transmission can be successfully completed. Therefore, even with the idle times set in the terminal stations being moved up one by one for resetting if normal transmission has been successfully completed by a specific terminal station, it is not possible to start transmission before the time allocated based on the timing signal (refer to FIG. 26). That is, the time period supposed to be used by a specific terminal station can never be used by another terminal station.
Therefore, an object of the present invention is to provide a medium access control method and system for scheduling a data transmission time period of each terminal station by transmitting one specific frame from a control station to each terminal station and for allowing a time period unused by a terminal station for data transmission to be allocated to another terminal station.